Several cellular pathways involved in RNA-mediated gene suppression have been described, each distinguished by a characteristic pathway and specific components. See, for example, the reviews by Brodersen and Voinnet (2006), Trends Genetics, 22:268-280, and Tomari and Zamore (2005) Genes & Dev., 19:517-529. The siRNA pathway involves the non-phased cleavage of a double-stranded RNA (“RNA duplex”) to small interfering RNAs (siRNAs). The microRNA pathway involves microRNAs (miRNAs), non-protein coding RNAs generally of between about 19 to about 25 nucleotides (commonly about 20-24 nucleotides in plants) that guide cleavage in trans of target transcripts, negatively regulating the expression of genes involved in various regulation and development pathways. Plant miRNAs have been defined by a set of characteristics including a stem-loop precursor that is processed by DCL1 to a single specific ˜21-nucleotide miRNA, expression of a single pair of miRNA and miRNA* species from the RNA duplex with two-nucleotide 3′ overhangs, and silencing of specific targets in trans. See Bartel (2004) Cell, 116:281-297; Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385; Jones-Rhoades et al. (2006) Annu. Rev. Plant Biol., 57:19-53; Ambros et al. (2003) RNA, 9:277-279. In the trans-acting siRNA (ta-siRNA) pathway, miRNAs serve to guide in-phase processing of siRNA primary transcripts in a process that requires an RNA-dependent RNA polymerase for production of an RNA duplex; trans-acting siRNAs are defined by lack of secondary structure, an miRNA target site that initiates production of double-stranded RNA, requirements of DCL4 and an RNA-dependent RNA polymerase (RDR6), and production of multiple perfectly phased ˜21-nucleotide small RNAs with perfectly matched duplexes with two-nucleotide 3′ overhangs (see Allen et al. (2005) Cell, 121:207-221).
MicroRNAs (miRNAs) are non-protein coding RNAs, generally of between about 19 to about 25 nucleotides (commonly about 20-24 nucleotides in plants), that guide cleavage in trans of target transcripts, negatively regulating the expression of genes involved in various regulation and development pathways (Bartel (2004) Cell, 116:281-297). In some cases, miRNAs serve to guide in-phase processing of siRNA primary transcripts (see Allen et al. (2005) Cell, 121:207-221).
Some microRNA genes (MIR genes) have been identified and made publicly available in a database (“miRBase”, available on line at microrna.sanger.ac.uk/sequences). The applicants have disclosed novel MIR genes, mature miRNAs, and miRNA recognition sites in U.S. patent application Ser. No. 11/303,745, filed 15 Dec. 2005, which are incorporated by reference herein. Additional MIR genes and mature miRNAs are also described in U.S. Patent Application Publications 2005/0120415 and 2005/144669A1, which are incorporated by reference herein. MIR genes have been reported to occur in intergenic regions, both isolated and in clusters in the genome, but can also be located entirely or partially within introns of other genes (both protein-coding and non-protein-coding). For a recent review of miRNA biogenesis, see Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385. Transcription of MIR genes can be, at least in some cases, under promotional control of a MIR gene's own promoter. MIR gene transcription is probably generally mediated by RNA polymerase II (see, e.g., Aukerman, and Sakai (2003) Plant Cell, 15:2730-2741; Parizotto et al. (2004) Genes Dev., 18:2237-2242), and therefore could be amenable to gene silencing approaches that have been used in other polymerase II-transcribed genes. The primary transcript (which can be polycistronic) is termed a “pri-miRNA”, a miRNA precursor molecule that can be quite large (several kilobases) and contains one or more local double-stranded or “hairpin” regions as well as the usual 5′ “cap” and polyadenylated tail of an mRNA. See, for example, FIG. 1 in Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385.
In plant cells, microRNA precursor molecules are believed to be largely processed in the nucleus. The pri-miRNA is processed to a shorter miRNA precursor molecule that also includes a stem-loop or fold-back structure and is termed the “pre-miRNA”. In plants, miRNAs and siRNAs are formed by distinct DICER-like (DCL) enzymes, and in Arabidopsis a nuclear DCL enzyme (DCL1) is believed to be required for mature miRNA formation; see, for example, Ambros et al. (2003) RNA, 9:277-279, and Xie et al. (2004) PLoS Biol., 2:642-652. Additional reviews on microRNA biogenesis and function are found, for example, in Bartel (2004) Cell, 1116:281-297; Murchison and Hannon (2004) Curr. Opin. Cell Biol., 16:223-229; and Dugas and Bartel (2004) Curr. Opin. Plant Biol., 7:512-520. MicroRNAs can thus be described in terms of RNA (e.g., RNA sequence of a mature miRNA or a miRNA precursor RNA molecule), or in terms of DNA (e.g., DNA sequence corresponding to a mature miRNA RNA sequence or DNA sequence encoding a MIR gene or fragment of a MIR gene or a miRNA precursor).
MIR gene families are estimated to account for 1% of at least some genomes and capable of influencing or regulating expression of about a third of all genes (see, e.g., Tomari et al. (2005) Curr. Biol., 15:R61-64; G. Tang (2005) Trends Biochem. Sci., 30:106-14; Kim (2005) Nature Rev. Mol. Cell. Biol., 6:376-385). Because miRNAs are important regulatory elements in eukaryotes, including animals and plants, transgenic suppression of miRNAs could, for example, lead to the understanding of important biological processes or allow the manipulation of certain pathways (e.g., regulation of cellular differentiation, proliferation, and apoptosis) useful, for example, in biotechnological applications. See, for example, O'Donnell et al. (2005) Nature, 435:839-843; Cai et al. (2005) Proc. Natl. Acad. Sci. USA, 102:5570-5575; Morris and McManus (2005) Sci. STKE, pe41 (stke.sciencemag.org/cgi/reprint/sigtrans;2005/297/pe41.pdf). MicroRNA (MIR) genes have identifying characteristics, including conservation among plant species, a stable foldback structure, and processing of a specific miRNA/miRNA* duplex by Dicer-like enzymes (Ambros et al. (2003) RNA, 9:277-279). These characteristics have been used to identify miRNAs and their corresponding genes in plants (Xie et al. (2005) Plant Physiol., 138:2145-2154; Jones-Rhoades and Bartel (2004) Mol. Cell, 14:787-799; Reinhart et al. (2002) Genes Dev., 16:1616-1626; Sunkar and Zhu (2004) Plant Cell, 16:2001-2019). Publicly available microRNA genes are catalogued at miRBase (Griffiths-Jones et al. (2003) Nucleic Acids Res., 31:439-441).
MiRNAs are expressed in very specific cell types in Arabidopsis (see, for example, Kidner and Martienssen (2004) Nature, 428:81-84, Millar and Gubler (2005) Plant Cell, 17:705-721). Suppression can be limited to a side, edge, or other division between cell types, and is believed to be required for proper cell type patterning and specification (see, e.g., Palatnik et al. (2003) Nature, 425:257-263). Suppression of a GFP reporter gene containing an endogenous miR171 recognition site was found to limit expression to specific cells in transgenic Arabidopsis (Parizotto et al. (2004) Genes Dev., 18:2237-2242). Recognition sites of miRNAs have been validated in all regions of an mRNA, including the 5′ untranslated region, coding region, and 3′ untranslated region, indicating that the position of the miRNA target site relative to the coding sequence may not necessarily affect suppression (see, e.g., Jones-Rhoades and Bartel (2004). Mol. Cell, 14:787-799, Rhoades et al. (2002) Cell, 110:513-520, Allen et al. (2004) Nat. Genet., 36:1282-1290, Sunkar and Zhu (2004) Plant Cell, 16:2001-2019).
The mature miRNAs disclosed herein are processed from MIR genes that generally belong to canonical families conserved across distantly related plant species. These MIR genes and their encoded mature miRNAs are also useful, e.g., for modifying developmental pathways, e.g., by affecting cell differentiation or morphogenesis (see, for example, Palatnik et al. (2003) Nature, 425:257-263; Mallory et al. (2004) Curr. Biol., 14:1035-1046), to serve as sequence sources for engineered (non-naturally occurring) miRNAs that are designed to silence sequences other than the transcripts targetted by the naturally occurring miRNA sequence (see, for example, Parizotto et al. (2004) Genes Dev., 18:2237-2242; also see U.S. Patent Application Publications 2004/3411A1 and 2005/0120415, incorporated by reference herein), and to stabilize dsRNA. A MIR gene itself (or its native 5′ or 3′ untranslated regions, or its native promoter or other elements involved in its transcription) is useful as a target gene for gene suppression (e.g., by methods of the present invention), where suppression of the miRNA encoded by the MIR gene is desired. Promoters of MIR genes can have very specific expression patterns (e.g., cell-specific, tissue-specific, or temporally specific), and thus are useful in recombinant constructs to induce such specific transcription of a DNA sequence to which they are operably linked.
This invention provides novel microRNAs and microRNA precursors identified from plants (including crop plants such as maize, rice, and soybean), as well as recombinant DNA constructs including such novel miRNAs, miRNA precursors, miRNA recognition sites, miRNA decoy sequences, and miRNA promoters corresponding to the miRNAs. Also disclosed and claimed are non-natural transgenic plant cells, plants, and seeds containing in their genome a recombinant DNA construct of this invention. Further provided are methods of gene suppression using recombinant DNA constructs of this invention and methods of providing transgenic plants with desired phenotypes, especially transgenic plants exhibiting increased yield (relative to non-transgenic plants) under abiotic stress conditions including drought, nutrient deficiency, and cold or heat stress.